Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500–600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20–30% at wavelengths of 900–1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >1012 Jones.

This manuscript reports that protonated amine interlayer induces an electrical doping with polymer:nonfullerene active layer and therefore yields narrowband NIR photodetectors with 500-600 nm active layers. The PEIE/PDIN leads to narrowband spectra while ZnO/PFN-Br doesn't. The phenomenon is ascribed to the positive space charge induced by the doping. The finding is interesting and the explanation is reasonable. I would suggest its publication in Nat Commun. The following comments should be elaborated: 1. Why the amines have to be protonated to get the effective doping and narrow the spectral response?
Please clarify it in the manuscript.
This comment is very relevant and gave us the chance to re-consider the role of protonated-amine in narrowing the spectral response. To better understand the doping mechanism, we did more supplementary experiments (XPS and EQE) and reconsidered our understanding of the function of the amine protonation. In fact, once the amine is fully protonated, no lone-pair electron on the nitrogen can participate in electron transfer to the NFA and thus no n-doping effect would occur. Only amine-ETL with non-tethered lone-pair electrons or electrons from quaternary ammonium anions can induce an ntype doping of non-fullerene active layers. We moved Fig. 2d and Fig.S16 to the revised Fig.3 and rewrote this section. Please see the revised Fig.3 and relevant discussion in the revised manuscript (pages 9-11), "In order to further confirm n-type doping and elucidate on its origin in the PEIE or PDIN devices, we performed high-resolution X-ray photoelectron spectroscopy (XPS) on the different types of amine-ETL coated ITO substrates and track the position and shape of the N1s peak, shown in Fig. 3a-c. We first consider the N1s spectrum ( Fig.3a) of PEIE (processed from H2O) which contains two main peaks from neutral anime nitrogen located at 400 and 400.9 eV, and a relatively small peak at a higher binding energy position of 401.8 eV indicative of protonated amine (N + ) in line with previous N1s XPS studies 48,49 . The N1s peak at the lower binding energy (400 eV) is ascribed to N-CH2-CH2OH tertiary amine due to the attached electron-rich alcohol hydroxyl end groups. To understand which neutral amine group is the main source for electron-donating or n-doping, we processed PEIE from water with different pH values since acidic conditions are considered to promote the protonation of amine while basic conditions suppress it 49 . Indeed, we observe that the N1s spectrum (Fig. 3a, Top) peaked at 400.9 eV vanishes and shifts to 401.8 eV at low pH conditions of pH ~ 4, implying that the (CH2)2-NH-secondary amines in PEIE are fully protonated. As a result, such protonated-PEIE device shows a broadband EQE, indicating a significant reduction in n-doping density. On the contrary, protonation of PEIE can be completely suppressed when processed from a basic solution (PH~12): No N + peak is observed in N1s spectrum, resulting in a narrowband response with lower EQE values indicative of an increased n-doping concentration. These results verify that the number of lone-pair on the secondary amine determines the n-doping ability, and also confirm that protonated-amine has no lone-pair to donate free electron to the NFA for n-doping. The proposed mechanism of electron transfer pathways between PEIE and NFA is shown in Fig. 3g. In short, to ensure PEIE as an n-dopant, it should be processed from a non-acidic solution.
We then turn to PFN which has tertiary amines on the side-chain. In principle, it can donate an electron from the lone-pair on the nitrogen of the tertiary amines (N1s peak at ~400 eV in Fig. 3b), and previous literature reports have shown its ability to n-dope NFAs 50 . However, the PFN device shows a rather broadband EQE (Fig. 3e) indicating a much weaker doping compared to the (CH2)2-NH-amine group containing PEIE interlayers. Only when we mixed PFN directly into the blend at an 1:50 weight ratio, spectral narrowing is visible. When PFN is converted to PFN-Br, the tertiary amine loses the lone-pair electron and forms a quaternary ammonium salt with Br -, resulting in no free electrons that can be transferred to NFA, and thus a broadband EQE (Fig. 2a). However, this situation changes once a strong electron-accepting unit such as a perylene diimide (PDI) core is present in the molecular structure. This is the case for PDIN and its derivative PDINO, which are also called self-doped n-type materials 51,52,53,54,55 . Self-doping of these materials is verified by the UV-VIS-NIR measurements ( Fig.S20) showing a broad sub-gap absorption band from 600 to 1600 nm which can be assigned to the polaronic transition (see ref. 53). In these materials, the tethered tertiary amine (PDIN) and ammonium salt (PDINO) can release free lone-pair electrons and electrons from the Oanions bonded to the quaternary amine, respectively, by intramolecular electron transfer 51,54 . As a result, devices containing such self-doped ETLs show narrowband EQE spectra. The spectra of these devices are even narrower than those with the PEIE interlayer, which indicates a more efficient electron-transfer from ETL to NFA.
For a similar PDI acceptor (N2200), which has only imide-N in the molecular structure, the lone-pair electron on the amide (imide-N) is delocalized between the nitrogen and the neighbour oxygen through resonance, and can therefore not contribute to n-type doping. Therefore, the EQE of the device using N2200 blended within the active layer, is broadband (Fig. 3f). Fig. 3g summarizes the above discussion: The efficiency of electron transfer or n-doping is highest for self-doping ETLs, followed by secondary amine containing ETLs and tethered tertiary amine containing ETLs. For ammonium salt containing ETLs, the n-type doping effect is the weakest.  Chem. Sci. 7, 1914-1919(2016 In addition, the title and the relevant sentences in abstract, introduction, and conclusion have been revised accordingly. Title: "Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection ˮ Abstract: "nonfullerene blends. We show that a protonated-electron-donating electron amine-interlayers can induce ˮ Introduction: "layers is simply achieved through applying an protonated electron-donating amineinterfacial layer ˮ Conclusion: "were intentionally n-doped by electron-donating amine-interlayers. ˮ  . Please keep in mind that only the carriers in the SCR can be collected in the doped device and contributed to EQE. The second reviewer asked a similar question (see comment 1). We revised Fig. S5, and simulate how the doping concentration (or the width of SCR) and active layer thickness affect the FWHM of the narrowband response.
3. There are self-doping ETL in the literature. Will they help to the obtain the narrow band?
From the above discussion in the comment 1, we believe that literature reported self-doped ETLs (most of them are derivatives of PDIN, see ref. 51-55) can induce n-type doping of NFA blends. So, with a sufficient thick junction (~500 nm), self-doped ETLs will indeed help to obtain a narrowband spectral response.

As for the doping, what depth is the doping between the ETL and the active layer?
The depth of doping is homogeneous throughout the whole blend film, as indicated by the Mott-Schottky analysis. Please see the apparent doping distribution derived via Mott-Schottky analysis in Figure S14 in the revised Supplementary Information. "Mott−Schottky plot over a 2.5 V range, which translates in a comparably flat doping profile (Fig. S14), indicating that the depth of n-doping is homogeneous throughout the whole blend film. The corresponding slope yields the doping concentration calculated viaˮ More evidences are needed to confirm the electrical doping, i.e., increase of electrical conductivity?
As suggested by the reviewer, we performed electrical conductivity measurement on the ETL-dependent devices in a configuration of ITO/ETL/PCE10:COTIC-4F (500 nm)/Al, which is an electron-only device.
The measured J-V curves are shown below. Indeed, doped devices (PDIN, PEIE) have a higher current density than non-doped devices (PFN-Br, ZnO), indicating improved electrical conductivity by n-doping.
In the revised manuscript (page 9): "Such n-doping between ETL and NFA active layer is also supported by the observation of an improved electrical conductivity (Fig. S15). ˮ Figure S15. Semi-logarithmic current density versus voltage characteristics of ETL-dependent electrononly devices, measured in the dark.
5. If the top electrode is transparent (thin silver for example), what will the spectral response is when illuminated from the HTL side?
We expect that the spectral response would be broadband when illuminated from the HTL side since the majority of carriers are in that case generated near the semi-transparent thin-Ag anode where a large electric field is present. To prove it, we fabricated such thin-Ag device with the structure of ITO/PDIN/PCE10:COTIC-4F (500 nm)/MoO3/Ag (15 nm). When illuminating the PDIN-device from the anode side, the measured EQE turns to be broadband and the shape follows the transmission of thin-Ag electrode, as shown in the below Figure. In the revised manuscript (page 7): "measurements in Figs. S9 and S10), resulting in a broadband EQE spectrum even if the blend thickness increases to ~1 μm (Fig. S11). ˮ The manuscript by Liu et al. entitled "Protonated aminie-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection" has presented a new detector design to achieve narrowband near-infrared response. The design involves incorporating amine interlayers near the anode. This approach is novel and less materials dependent and have been shown to work for 3 different blends, demonstrating the generalizability of this concept. While the device performance is excellent, there are some clarification requests as listed below: 1. The paper has shown that doping concentration is significant different with the use of PEIE and PDIN interface. Would there be a knob to change the doping concentration more precisely in the future, Yes, there is. By quantitatively controlling the amount of PDIN in the NFA-blend, the n-doping concentration can be manipulated more precisely. We did the experiment of adding PDIN (8mg/mL, prepared in Chloroform+0.5 vol% CH3COOH) into the PCE10: COTIC-4F (40 mg/mL) solution with a volume ratio of 1:20, and n-doping induced spectral narrowing was observed as shown by the EQE spectrum below (and Fig.S22a in the revised SI). However, PDIN tends to aggregate in the blend even when added in small quantities, ultimately altering the morphology of the solid-state blend film. As a result, the measured dark current under reverse bias is increased by more than four orders of magnitude ( Fig.S22b). In future work, the electron-donating interfacial material should be carefully selected to ensure an appropriate n-doping density without significantly altering the blend morphology and charge transporting properties.
In the revised manuscript (page 12): "The measured EQEs correspond to a larger w of ~ 250 nm, confirming the weaker doping of PEIE as compared to PDIN (see Nt in Table S1). A more precise control of the n-type doping concentration can be realized by adding a small, known amount of such electron-donating interfacial material in the NFA blend. Care must however be taken to ensure that the n-dopant does not significantly alter the blend morphology and charge transport properties, otherwise the dark current of the resulting narrowband OPD increases dramatically (Fig. S22). ˮ To clarify, we add relevant discussion in the revised manuscript (page 6).
"EQEs in Fig. S7). We note that for a typical 500 nm-thick junction, a further decrease in the width of the SCR (w) or a higher n-doping concentration does not further reduce the FWHM. However, increasing the active layer thickness does (see FWHM of the simulated narrowband spectra as function of w and blend thickness in Fig.S5 c-f). A FWHM of ~30 nm for PCE10:COTIC-4F narrowband OPD is theoretically achievable for an 1100 nm-thick blend.ˮ 2. In the inset of Fig 4a, there is a dip in the noise spectra in the beginning. Would the authors explain why it is the case? It is not typical and opposite to the usual 1/f trend.
We thank the reviewer for this comment. To address the reviewer's concern, we have thoroughly revisited the experimental noise current density (NSD) spectra measurements and found the dips in noise spectra at low frequency to be related to the used current pre-amplifier when operating at high gain and low frequency. We would like to note that the specific detectivities were calculated based on noise currents obtained from NSD spectra at high frequency (i.e., >500 Hz) and were therefore not affected by the dips in noise spectra. Based on these findings, we carefully repeated the NSD measurements using an electrical noise-minimized and current pre-amplifier-free setup. As a result, the  4. There are a lot of acronyms, making it somewhat difficult to follow, and there is one acronym SCCN that was undefined on p10, but I think it's spectral charge collection narrowing? Anyways, it might be more readable if some acronyms are spelled out in words if possible.
We are sorry for forgetting to remove the acronym SCCN in the submitted version, which was defined as space charge collection narrowing in the earlier draft. We thought it would not be suitable and decided to removed it. Please see the revised manuscript (page12).
"Having established the mechanism of n-doping enabled SCCN spectral narrowing in the NIR photodiodes, we now evaluate their photo-detection performance. ˮ 1) The authors claimed that the n-doping effect and consequentially the SCR is induced by the ETL.
The authors supported this statement by indicating that trap-assisted recombination is not playing a role by carrying out light-intensity JVs ( Figure S8). I believe that further measurements are necessary to exclude trap-assisted recombination, i.e. charge extraction and transient photovoltage measurements.
Measurements of Voc vs intensity are a reliable way to study trap-assisted recombination in organic photovoltaics. Indeed, this is what we have done and shown in Figure S8 ( Figure S12 in the revised supplementary information). TPV and charge extraction measurements do not contain more information on trap assisted recombination, and moreover, interpretation of those measurements is difficult for doped layers. Up to now, when those measurements are done in the literature, no doping is assumed.
This makes it really hard to draw more meaningful conclusions on surface trap assisted-recombination from such measurements. We therefore refrained from adding such measurements to the manuscript.
We have however, in this revised manuscript, considerably strengthened the evidence for n-doping being the ETL (please see answers to previous referee questions).
2) In addition, a common method to study doping is EPR. I strongly recommend the authors to carry out this measurement.
We agree with the review that EPR is a good tool to study the doping for organic semiconducting materials, but we and our collaborators do not have access to such a setup at this moment. However, to see the doping effect and calculate the doping density of the device, capacitance-voltage measurement is more reliable and meaningful (refer to Fig. 2c and Fig. S20 containing this measurement). In addition, We cite relevant papers that show EPR data for the self-doping n-type ETL and n-doping NFA blend films, to further support the interlayer can induce an n-type doping of NFA.
3) The morphology of the active layer can be drastically influenced by the layer below. I suggest performing contact angle measurements for active layers coated on different ETL to exclude this variable.
Following the reviewer's suggestion, we performed contact angle measurement on PCE10: COTIC-4F blend films coated on the studied PFN-Br, ZnO, PEIE and PDIN interfacial layers. As shown in the revised Fig. 8a, all samples have a very similar contact angle (CA, water) closed to that of neat polymer film of 99 o , indicating a donor polymer-rich top surface. In addition, we also performed atomic force microscope (AFM) measurements on such different ETL coated blend films, shown in Fig S8b. Again, no obvious difference in root mean square (RMS) roughness and significant morphology change is observed amongst those samples. Then, it would be safe to exclude severe morphological changes of the active layer when different ETLs are applied. In the revised manuscript (Page 7): "Note that the morphology of the active layer is negligible influenced by the electron-transporting interfacial layer below, which is confirmed by atomic force microscope (AFM) and static water contact angle measurements on the different ETL-coated blend films (Fig. S8).ˮ 4) In some figures, the active layer thickness is missing.
We have checked all the figures in the main text, and the active layer thickness in Fig.5